1. Field of the Invention
The present invention relates to an apparatus for fixing electronic parts to a printed circuit board or substrate, and more particularly to an apparatus suitable for attaching to a high density printed circuit board a variety of electronic parts such as resistors, capacitors, and so-called flat packaged elements from which four-way electrode terminals are extracted in a planar manner.
2. Description of the Prior Art
Recently, a higher density attachment of electronic parts to a printed circuit board has been developed. Since a soldering or brazing operation to attaching electronic parts such as semiconductor chips to a printed circuit board is attained in a final step of the working line, a performance of the electronic parts largely depends upon a quality of the soldering work. Thus, the soldering technique becomes the most important and significant technique in various works in the manufacturing line.
At present, a soldering apparatus has been proposed which employs a vapor having a large specific weight relative to that of air as thermal medium and heats material to be processed by utilizing its condensing latent heat, in view of needs to enhance a uniformity of temperature distribution within a soldering work furnace and to avoid an undesirable excessive heating of the electronic parts. More specifically, the thermal medium used in such a vapor tank generally comprises fluorine system inactive organic agent. Its saturated vapor may have a large specific weight (relative to that of the air) of about 20 times, with its molecular weight being at about 820 g/mol under an intended temperature/pressure condition.
Thus, the thermal medium is very expensive, and variety of collecting means for collecting and reusing the vapor are proposed.
In this apparatus, the electronic parts are mounted on a soldering pattern on the printed circuit board, and the printed circuit board is caused to pass through saturated vapor of a thermal medium having a large specific weight relative to that of air as described above, by delivering means such as a conveyor, so that the solder on the board is heated and molten. As a result, the electronic parts are soldered on the printed circuit board. Thus, the apparatus has a vapor tank which is a so-called vapor reflow reservoir.
Such an apparatus suffers from the following disadvantages.
(1) When the conveyor passing through the cooling chamber is again introduced into a preheating chamber, a liquefied thermal medium stuck to the conveyor is heated by a preheater so that a part of the thermal medium is vaporized. When the vapor of the thermal medium is brought into contact with a heating surface of the preheater, a poisonous gas would be generated. Thus, there is a safety problem.
(2) A part of the thermal medium condensed and liquefied within the vapor generating tank or reservoir is adhered to th solder member to be soldered as the conveyor and is introduced together therewith into the cooling chamber, so that the part of the thermal medium is stagnant on the lower side of the cooling chamber or discharged to the outside. Since the liquefied thermal medium has a large density even if its volume is small, the total weight of the liquefied thermal medium is much larger than that of the vapor thereof. In addition, the thermal medium is very expensive, which increases a running cost and has an adverse effect in economical aspect.
Incidentally, such conventional apparatus is disclosed in, for example, U.S. Pat. No. 4,389,797.